Internal combustion engines that are operated either constantly or at times with a lean air-fuel mixture produce nitrogen oxides NOx (mainly NO2 and NO), which require NOx-reducing measures. Exhaust gas recirculation (EGR) is an engine-related measure aimed at reducing NOx raw emissions in the exhaust gas, and this is a process in which part of the exhaust gas of the internal combustion engine is recirculated into its combustion air. As a result, the combustion temperatures are lowered, and consequently, the formation of NOx (NOx raw emissions) is reduced. As a rule, however, exhaust gas recirculation alone is not sufficient to comply with statutory NOx limit values, which is why there is an additional need for an active exhaust gas after-treatment that lowers the NOx final emissions by catalytically reducing NOx to form nitrogen N2. A known NOx exhaust gas after-treatment involves the use of NOx storage catalysts. During lean operation (λ>1), these catalysts store nitrogen oxides in the form of nitrates. During short rich intervals (λ<1), the stored nitrogen oxides are once again desorbed and reduced to nitrogen N2 in the presence of the reductants present in the rich exhaust gas.
Another approach for converting nitrogen oxides in the exhaust gases of lean-burning internal combustion engines is the use of catalyst systems that operate according to the principle of selective catalytic reduction (SCR). These systems comprise at least one SCR catalytic converter that, in the presence of a reductant metered into the exhaust gas—normally ammonia NH3—selectively convert the nitrogen oxides of the lean exhaust gas into nitrogen and water. In this context, the ammonia can be metered into the exhaust gas stream from an aqueous solution of ammonia. Normally, however, it is obtained from a precursor compound, for instance, urea in the form of an aqueous solution or solid pellets, through the modality of thermolysis and hydrolysis.
It likewise a known procedure to continuously check the proper functioning of exhaust gas catalytic converters such as, for example, SCR catalytic converters or NOx-storage catalytic converters by means of on-board diagnostics (OBD). Towards this end, the signal from an exhaust gas sensor located downstream from the catalytic converter is normally employed for the appertaining exhaust gas component in order to measure the concentration of this exhaust gas component downstream from the exhaust gas catalytic converter. Furthermore, the concentration of the exhaust gas component is determined upstream from the catalytic converter, that is to say, the raw emissions from the engine. This can be done by measuring the concentration by means of another exhaust gas sensor installed upstream from the catalytic converter. However, the raw emissions are more often ascertained by means of modeling using stored characteristic maps that depict the concentration of the component as a function of a momentary operating point of the internal combustion engine. The status of the exhaust gas catalytic converter in terms of its conversion of the component is a function of the concentration measured downstream from the catalytic converter and of the measured or modeled concentration upstream from the catalytic converter, that is to say, the raw emissions of the internal combustion engine. The status of the exhaust gas catalytic converter can be calculated, for example, in the form of the efficiency η in accordance with the equation below, wherein c2 is the concentration (or the content) of the exhaust gas component measured downstream from the catalytic converter and c1 stands for the raw emissions of this component by the internal combustion engine:
  η  =      1    -                  c        ⁢                                  ⁢        2                    c        ⁢                                  ⁢        1            
The efficiency η can thus assume values from 0 to 1. An ideally functioning catalytic converter that brings about a complete catalytic conversion (c2=0) thus exhibits an efficiency η of 1, whereas η=0 (c1=c2) in the case of a completely inactive catalytic converter.
German patent application DE 10 2010 042 442 A1 describes an exhaust gas system with an SCR catalytic converter as well as with a low-pressure (LP) exhaust gas recirculation system by means of which a partial stream of exhaust gas is withdrawn downstream from a turbine (on the low-pressure side) of an exhaust gas turbocharger and downstream from the SCR catalytic converter, while the combustion air of the internal combustion engine is fed in upstream from a compressor (on the low-pressure side) of the exhaust gas turbocharger. In the exhaust gas recirculation line, there is an NOx sensor that measures the concentration of nitrogen oxides in order to regulate the internal combustion engine on the basis of the NOx concentration thus ascertained, especially the exhaust gas recirculation (EGR) rate or the air-fuel ratio. The exhaust gas recirculation line is also connected to the air line of the internal combustion engine via a bypass that opens up into the air line downstream from the compressor, in other words, on its high-pressure side. In order to determine an offset of the NOx sensor so that it can be calibrated, the bypass is opened, which brings about a reversal in the direction of flow in the exhaust gas recirculation line, so that the NOx sensor is charged with fresh air. A diagnose of the SCR catalytic converter is not described here.
German patent application DE 10 2010 050 413 A1 discloses an exhaust gas system of an internal combustion engine that has an oxidation catalytic converter, a particulate filter as well as an SCR catalytic converter, whereby optionally, the particulate filter can have an SCR catalytic coating, and a separate SCR catalytic converter can be dispensed with. Moreover, the internal combustion engine has a double charge with a combination of high-pressure (HP) and low-pressure (LP) exhaust gas recirculation (EGR) in which the low-pressure EGR line branches off downstream from the particulate filter and it accommodates an additional SCR catalytic converter.
German patent application DE 103 49 126 A1 describes the above-mentioned approach for ascertaining the efficiency of an SCR catalytic converter, either by means of an NOx sensor upstream as well as downstream from the catalytic converter or else, instead of the upstream sensor, by means of a mathematical model for the NOx raw emissions. If the calculated NOx efficiency of the SCR catalytic converter falls below a prescribed basic efficiency, the catalytic converter is regenerated, eliminating the deposited hydrocarbons and soot.
Both of the above-mentioned approaches for diagnosing an exhaust gas catalytic converter entail drawbacks. The arrangement of an exhaust gas sensor upstream as well as downstream from the catalytic converter constitutes a costly measure involving additional equipment. If, in contrast, the upstream exhaust gas sensor is replaced by a mathematical model for ascertaining the raw emissions of the pertinent exhaust gas component, the diagnostic robustness can be detrimentally affected due to unavoidable imprecision or unreliability of the mathematical model employed. Moreover, the use of such a model is associated with extra costs.